Friction Evolution Research Articles

Influence of intensive parameters and assemblies on friction evolution during piston-cylinder experiments

Abstract Piston-cylinder assemblies exhibit inhomogeneous pressure distributions and biases compared to the theoretical pressure applied to the hydraulic press because of the thermal and mechanical properties of the assembly components. Whereas these effects can partially be corrected by conventional calibration, systematic quantification of friction values remain very sparse and results vary greatly among previous studies. We performed an experimental study to investigate the behavior of the most common cell assemblies, i.e., talc [Mg3Si4O10(OH)2], NaCl, and BaCO3, during piston-cylinder experiments to estimate the effects of pressure, temperature, run duration, assembly size, and assembly materials on friction values. Our study demonstrates that friction decreases with time and also partially depends on temperature but does not depend on pressure. We determined that friction decreases from 24 to 17% as temperature increases from 900 to 1300 °C when using talc cells, indicating a friction decrease of ~2% per 100 °C increase for 24 h experiments. In contrast, friction becomes independent of time above 1300 °C. Moreover, at a fixed temperature of 900 °C, friction decreases from 29% in 6 h runs to 21% in 48 h runs, corresponding to a decrease of friction of 0.2% per hour. Similar results obtained with NaCl cell assemblies suggest that friction is constant within error, from 8% in 9 h runs to 5% in 24 h runs. At 900 °C, possible steady-state friction values are only reached after at least 48 h, indicating that friction should be considered a variable for shorter experiments. We establish that assembly materials (and their associated thermomechanical properties) influence the friction correction more than the dimensions of the assembly parts. Finally, we show that the use of polytetrafluoroethylene film instead of conventional Pb foil does not modify friction but significantly reduces the force required for sample extraction, thus increasing the lifetime of the carbide core, which in turn enhances experimental reproducibility.

Model pile behavior in calibration chamber under very large number of cycles of axial loading in saturated clay

ABSTRACT The results of a series of displacement-controlled cyclic axial loading tests of model pile in saturated kaolinite clay, carried out in a calibration chamber, are presented. An instrumented pile-probe was used to investigate the evolution of local skin friction and excess pore water pressure during cyclic loading. After a brief description of the experimental setup and the testing procedure, the tests results are presented concerning the response of the probe during installation, initial monotonic axial loading, cyclic loading up to large numbers of cycles (105 cycles) and post-cyclic monotonic axial loading. During cyclic loading, an initial degradation phase of local skin friction followed by a stabilization phase is observed. Generation of excess pore water pressure occurs during the 60 first cycles followed by a dissipation phase. Parametric study showed that, for initial monotonic loadings, maximum tip resistance and local skin friction are proportional to stress level. Furthermore, excess pore water pressure generated during cyclic loading depends both on initial stress level applied to the clay specimen and cyclic displacement amplitude. A peak value of local skin friction resistance has been observed during post-cyclic monotonic loadings.

Fault Friction During Simulated Seismic Slip Pulses

AbstractTheoretical studies predict that during earthquake rupture faults slide at non‐constant slip velocity, however it is not clear which source time functions are compatible with the high velocity rheology of earthquake faults. Here we present results from high velocity friction experiments with non‐constant velocity history, employing a well‐known seismic source solution compatible with earthquake source kinematics. The evolution of friction in experiments shows a strong dependence on the applied slip history, and parameters relevant to the energetics of faulting scale with the impulsiveness of the applied slip function. When comparing constitutive models of strength against our experimental results we demonstrate that the evolution of fault strength is directly controlled by the temperature evolution on and off the fault. Flash heating predicts weakening behavior at short timescales, but at larger timescales strength is better predicted by a viscous creep rheology. We use a steady‐state slip pulse to test the compatibility of our strength measurements at imposed slip rate history with the stress predicted from elastodynamic equilibrium. Whilst some compatibility is observed, the strength evolution indicates that slip acceleration and deceleration might be more rapid than that imposed in our experiments.

Effect of nano-modification and fiber-reinforcement on mechanical behavior of cement-stabilized dredged sediment

This study investigates the mechanical behavior of cement-stabilized dredged sediment (CDS) improved by nano-MgO (NM) and polypropylene fiber (PF). The effects of NM content, PF content and PF length on the mechanical behavior of NM-modified and PF-reinforced CDS (CNPDS) were evaluated by conducting unconfined compressive strength (UCS, qu ) and splitting tensile strength (STS, qt ) tests. Furthermore, optical microscope scans were conducted to detect the interface friction evolution. The results indicated that adding NM can significantly improve UCS and STS of CDS, and the optimum NM content was 1.2%. The optimum PF content tended to increase with increasing curing age, and the 3 mm-length PF was the most effective in improving UCS and STS. Increasing NM content mainly increased the strength, but had little influence on ductility. The inclusion of PF can effectively improve the ductility of CNPDS, and contributed to its tensile strain-hardening characteristic. The relation of was obtained regardless of other influence factors. The combination of NM-modification and PF induced “bridging” effect provided a coupling effect inside CNPDS, and a concept model representing this process was established. The key findings in this study can be used as an effective method for dredged sediment stabilization in practical engineering.

TiN-Ag as an antimicrobial and wear resistant coating

Nosocomial infections are a major clinical concern, posing great risks for patients and rising costs for health services providers. This work aims at developing a hard, wear resistant coating, whose antimicrobial properties shall prevent the transmission of infections.TiN coatings deposited by Physical Vapour Deposition, PVD, with different Ag contents have been studied, especially in relation to the hardness and adhesion, their microstructure and morphology. The antimicrobial activity of the surfaces has been assessed against Staphylococcus epidermidis at different time frames, one of the most troublesome source of infections in trauma and orthopaedic surgeries. The electro-tribology properties of different silver contest have been studied. Finally, the coatings have been deposited on surgical acetabular reamers and wear resistance tests have been carried out against synthetic composite bone (simulating cortical and cancellous bone).Results have shown a good coating adhesion on stainless steel (both quantitatively in the scratch tests and qualitatively in the tests against synthetic composite bone), while the hardness decreased with higher Ag percentages. Furthermore, coatings exhibited antimicrobial activity against S.epidermidis, limited silver release, a remarkable wear resistance (vs. uncoated surgical acetabular reamers), while the electrical contact resistance provided valuable information about the evolution of friction and the status of the coating. Therefore TiN-Ag coatings present promising features for reducing the risk of infections, monitoring and extending cutting edge life and quality, and thus limiting damage to living tissues, e.g. necrosis.

The variations of characteristic scale length of friction evolution affect earthquake dynamics

Within a fault governing model the characteristic scale length is one of the most relevant physical parameters because it accounts for the so–called fracture energy (density) of the system, its dynamics, the time during which the accumulated stress is released and the seismic waves are excited, the amount of slip developed during an instability event. Friction laboratory experiments reveal that it is not a material property, but that it changes with the sliding velocity. We propose two rather different analytical models to fit laboratory evidence and we incorporate them into a fault model able to simulate repeated earthquakes in the framework of various formulations of rate and state friction. We demonstrate that temporal variations of the scale length do not prevent the system to reach its limit cycle, but they systematically reduce the magnitude of the expected event (both in term of developed slip, and thus seismic moment, and released stress) and also reduce the inter–event time (recurrence interval). Depending on the friction model, the system can penetrate into the stable regime and can either continue the accelerating phase toward to failure or decelerate and abort instability.

Microscopic Mechanisms Behind the High Friction and Failure Initiation of Graphene Wrinkles.

Wrinkling occurs on the surfaces of large-area graphene ubiquitously. Despite that the wrinkled structures are found to degrade the lubricous property, the behind mechanisms remain less understood. Here, atomic force microscopy is adopted to characterize the friction and wear properties of graphene wrinkles (GWs) with different heights by nanoscratch tests. We verify the phenomena of high friction and reduced load-carrying capacity of wrinkles and report the observation of lubrication deterioration with increased heights. Using molecular dynamics simulations, we reveal that the contact quality at the interface is a dominant role in the friction evolution of wrinkles. The high friction of wrinkles is determined by the increased contact area and commensurability caused by the wrinkle deformation and topography changes. The wrinkle failure initiates near the root of the formed bilayer configuration due to the increased lateral stiffness and reduced atomic distance between the wrinkle layers. The increased interlocking effect results in a local shear stress of 91 GPa and induces the phase transitions of carbon atoms easily. As the wrinkle height decreases, the unstable local configuration weakens the interlocking effects and cannot fail even at a high load. This investigation sheds light on the microscopic frictional contact of GWs and provides guidance for tuning the tribological properties of graphene by controlling the wrinkle structures.

Evolution of adhesive wear and friction in elastic-plastic spherical contact

A finite element (FE) model is presented for the evolution of fracture and friction in elastic-plastic adhesive spherical contact. The model is based on a realistic criterion of ductile fracture in a three-dimensional contact problem and the failed elements are deleted. Full stick contact condition is used at the contact interface to model a strong adhesion there. Fracture evolution is presented allowing the process of material removal in the form of a wear particle. The evolution of friction with increasing tangential loading as well as sliding inception are presented. Predicted wear coefficient and static and dynamic friction coefficients are compared with experimental results and possible reasons for discrepancies between the predicted and experimental results are discussed.

Development of a novel high temperature open tribometer with laser-based heating system

Contacts operating under severe conditions such as high temperatures, contact pressures and/or sliding velocities, can occur in many applications and especially in metalworking processes. The extreme thermomechanical loadings withstood by the tooling during such interaction directly govern their lifetime but also the functional properties of the component surface. Understanding the friction and wear mechanisms behind is still an issue and can only be addressed with a relevant experimental set-up. This work presents the development of a new laser-based heating system fitted on an open tribometer with the aim of investigating such severe contact conditions for a AISI1045 carbon steel during sliding against uncoated cemented carbide. A 3D numerical approach is adopted to design the system and predict the temperature distribution, as well as the heating rates, below the laser beam and at the expected contact zone. A phase transformation model based on the simulated thermal kinetics is applied to identify the conditions leading to the potential formation of austenite and to show that a proper combination of laser power and sliding velocity has to be selected to ensure the desired contact conditions. It is shown for example that even at high sliding velocities, a maximum temperature of 600∘C can be reached in the contact zone without exceeding the melting temperature ahead while only a thin layer can be affected by the laser. The experimental set-up is described with the integration of a laser head into the test bench. An experimental campaign is carried out to show the influence of the temperature and sliding conditions on the tribological behavior especially in terms of friction, wear mechanisms and microstructural evolution beneath the contact track.

An experimental and molecular dynamics study of the superlubricity enabled by hydration lubrication

Recently, hydration lubrication has emerged as a promising approach to macroscale liquid superlubricity. However, the reported methods till now exhibit a long running-in process that features high and fluctuating friction evolution. Herein, we have employed glycerol aqueous solutions as the model to reveal the mechanism governing the achievement of superlow friction by hydration lubrication, aiming to erase the long and disturbing running-in. Through experimental investigations and molecular dynamics simulations, it is found that the initial high friction of dilute glycerol aqueous solution with the water/glycerol weight ratio of 2.0 is due to the ineffective film formation between the tribopairs, and the superlow low friction after the long running-in is attributed to the synergetic effect of increased viscosity due to water evaporation, increased film thickness due to higher speeds, reduced contact pressure due to the wear of friction materials, and the adsorbed molecular layers on steel surfaces. To improve the initial lubricating condition that is responsible for the high coefficient of friction at the start of the test, the formula has been modified with the water/glycerol weight ratio reduced to 0.2, and its friction performances have been further investigated. Surprisingly, remarkable and robust superlow friction appears almost as soon as the test starts. Molecular dynamics sliding simulation reveals the formation of shearing layers or hydration layers near the surfaces and the load-supporting layers between them. The results in this work are helpful to expand the understanding of the superlubricity mechanism enabled by hydration lubrication, and it also provides a novel strategy to liquid superlubricity using aqueous media that features vanished running-in.

Frictional properties and microstructural evolution of dry and wet calcite–dolomite gouges

Abstract. Calcite and dolomite are the two most common minerals in carbonate-bearing faults and shear zones. Motivated by observations of exhumed seismogenic faults in the Italian Central Apennines, we used a rotary-shear apparatus to investigate the frictional and microstructural evolution of ca. 3 mm thick gouge layers consisting of 50 wt % calcite and 50 wt % dolomite. The gouges were sheared at a range of slip rates (30 µm s−1–1 m s−1), displacements (0.05–0.4 m), and a normal load of 17.5 MPa under both room-humidity and water-dampened conditions. The frictional behaviour and microstructural evolution of the gouges were strongly influenced by the presence of water. At room humidity, slip strengthening was observed up to slip rates of 0.01 m s−1, which was associated with gouge dilation and the development of a 500–900 µm wide slip zone cut by Y-, R-, and R1-shear bands. Above a slip rate of 0.1 m s−1, dynamic weakening accompanied the development of a localised < 100 µm thick principal slip zone preserving microstructural evidence for calcite recrystallisation and dolomite decarbonation, while the bulk gouges developed a well-defined foliation consisting of organised domains of heavily fractured calcite and dolomite. In water-dampened conditions, evidence of gouge fluidisation within a fine-grained principal slip zone was observed at a range of slip rates from 30 µm s−1 to 0.1 m s−1, suggesting that caution is needed when relating fluidisation textures to seismic slip in natural fault zones. Dynamic weakening in water-dampened conditions was observed at 1 m s−1, where the principal slip zone was characterised by patches of recrystallised calcite. However, local fragmentation and reworking of recrystallised calcite suggests a cyclic process involving formation and destruction of a heterogeneous slip zone. Our microstructural data show that development of well-defined gouge foliation under the tested experimental conditions is limited to high velocities (>0.1 m s−1) and room humidity, supporting the notion that some foliated gouges and cataclasites may form during seismic slip in natural carbonate-bearing faults.

Fracture analyses of surface asperities during sliding contact

Fracture of interlocking asperities during sliding contact plays a central role in determining friction and wear of brittle materials. The local force to break surface asperities and subsurface crack propagation directly affect the frictional force, wear rate and evolution of surface roughness. Here, we study the fracture failure of an isolated asperity under a lateral contacting force using a systematic set of experiments and numerical simulations. Combining the generalized elastic fracture mechanics and the Euler-Bernoulli beam theory, we also develop an analytical model to predict the critical force at the onset of asperity fracture as a function of the asperity's shape and material properties. The developed model can be used to develop physics-based friction and wear models for brittle solids.

Experimental and modelling study of friction evolution and lubricant breakdown behaviour under varying contact conditions in warm aluminium forming processes

The lubricant behaviours under varying contact conditions were investigated by conducting pin-on-strip tests between P20 tool steel and AA7075 aluminium alloy using an automated testing system, Tribo-Mate. The effects of temperature changes, and rapid load and speed changes on the coefficient of friction (COF) and lubricant breakdown phenomenon were experimentally studied. The evolutions of COF showed three distinct stages, indicating the transformation from boundary lubrication condition to dry sliding condition. The value of COF at the initial stage was found to increase with increasing temperature. The increase of temperature, contact load and sliding speed caused an earlier breakdown of the lubricant. An interactive friction model was developed to predict friction evolutions at varying contact conditions. A close agreement with errors less than 6.8% were achieved between the model predictions and experimental results.

A molecular dynamics study of the oxidation mechanism, nanostructure evolution, and friction characteristics of ultrathin amorphous carbon films in vacuum and oxygen atmosphere

Amorphous carbon (a-C) films are characterized by extraordinary chemical inertness and unique thermophysical properties that are critical to applications requiring oxidation-resistant, low-friction, and durable overcoats. However, the increasing demands for ultrathin (a few nanometers thick) a-C films in various emerging technologies, such as computer storage devices, microelectronics, microdynamic systems, and photonics, make experimental evaluation of the structural stability and tribomechanical properties at the atomic level cumbersome and expensive. Consequently, the central objective of this study was to develop comprehensive MD models that can provide insight into the oxidation behavior and friction characteristics of ultrathin a-C films exhibiting layered through-thickness structure. MD simulations were performed for a-C films characterized by relatively low and high sp3 contents subjected to energetic oxygen atom bombardment or undergoing normal and sliding contact against each other in vacuum and oxygen atmosphere. The effect of energetic oxygen atoms on the oxidation behavior of a-C films, the dependence of contact deformation and surface attractive forces (adhesion) on surface interference, and the evolution of friction and structural changes (rehybridization) in the former a-C films during sliding are interpreted in the context of simulations performed in vacuum and oxidizing environments. The present study provides insight into the oxidation mechanism and friction behavior of ultrathin a-C films and introduces a computational framework for performing oxidation/tribo-oxidation MD simulations that can guide experimental investigations.

Prediction of Friction Degradation in Highways with Linear Mixed Models

The development of a linear mixed model to describe the degradation of friction on flexible road pavements to be included in pavement management systems is the aim of this study. It also aims at showing that, at the network level, factors such as temperature, rainfall, hypsometry, type of layer, and geometric alignment features may influence the degradation of friction throughout time. A dataset from six districts of Portugal with 7204 sections was made available by the Ascendi Concession highway network. Linear mixed models with random effects in the intercept were developed for the two-level and three-level datasets involving time, section and district. While the three-level models are region-specific, the two-level models offer the possibility to be adopted to other areas. For both levels, two approaches were made: One integrating into the model only the variables inherent to traffic and climate conditions and the other including also the factors intrinsic to the highway characteristics. The prediction accuracy of the model was improved when the variables hypsometry, geometrical features, and type of layer were considered. Therefore, accurate predictions for friction evolution throughout time are available to assist the network manager to optimize the overall level of road safety.

Evolution of contact performance of industry electrical connector based on reliability accelerated testing

The evolution of contact performance of electrical connectors, as an expression of service performance, played a significant role in various electronic equipment or system. However, very few methods had been used to detect the evolution of contact performance effectively and accurately. Hence, in this research, reliability accelerated testing was conducted to investigate the evolution of contact performance of electrical connectors. To detect the evolution of contact performance, contact resistance and friction and wear of the connector were measured using a DC resistance tester and an electron microscope respectively. Also, the effect of external conditions such as ambient temperature, mating speed, mating cycles was statistically investigated, and evolution curves were developed for contact resistance and abrasion loss. The obtained results revealed the temperature and mating speed affected the contact performance of electrical connectors. The increment of temperature reduced the shear strength of material and increased the thickness of oxide film. Increased mating speed greatly increased the probability of fracture of micro-protrusion due to collision, the wear form of connector had realized transition from low-speed adhesive wear to high-speed peeling wear. In addition, when the connector was mated about 3000 cycles, the contact performance of the connector would be greatly decreased.

Variations in strain affect friction and microstructure evolution in copper under a reciprocating tribological load

The microstructure of the materials constituting a metallic frictional contact strongly influences tribological performance. Being able to tailor friction and wear is challenging due to the complex microstructure evolution associated with tribological loading. Here, we investigate the effect of the strain distribution on these processes. High-purity copper plates were morphologically surface textured with two parallel rectangles—referred to as membranes—over the entire sample length by micro-milling. By keeping the width of these membranes constant and only varying their height, reciprocating tribological loading against sapphire discs resulted in different elastic and plastic strains. Finite element simulations were carried out to evaluate the strain distribution in the membranes. It was found that the maximum elastic strain increases with decreasing membrane stiffness. The coefficient of friction decreases with increasing membrane aspect ratio. By analyzing the microstructure and local crystallographic orientation, we found that both show less change with decreasing membrane stiffness.Graphic abstract

Evolution of dynamic shear strength of frictional interfaces during rapid normal stress variations

Pressure shear plate impact tests have revealed that when normal stress changes rapidly enough, the frictional shear resistance is no longer proportional to the normal stress but rather evolves with slip gradually. Motivated by these findings, we focus on characterizing the dynamic shear strength of frictional interfaces subject to rapid variations in normal stress. To study this problem, we use laboratory experiments featuring dynamic shear cracks interacting with a free surface and resulting in pronounced and rapid normal stress variations. As dynamic cracks tend to propagate close to the wave speeds of the material, capturing their behavior poses the metrological challenge of resolving displacements on the order of microns over timescales microseconds. Here we present our novel approach to quantify the full-field behavior of dynamic shear ruptures and the evolution of friction during sudden variations in normal stress, based on ultrahighspeed photography (at 1-2 million frames/sec) combined with digital image correlation. Our measurements allow us to capture the evolution of dynamic shear cracks during these short transients and enable us to decode the nature of dynamic friction.

Roughness and mechanical properties of electron-beam surface modified and TiN/TiO2-coated Ti6Al4V alloy

In this study, we used PVD deposition of TiN/TiO2 coatings on polished and electron-beam-treated Ti6Al4V alloy to explore the changes in the surface roughness and mechanical properties of the coated systems. After the electron-beam treatment (EBT), the average surface hardness and roughness (R a ) increased from 323±5.62 HV0.2 and 0.14±0.004 μm up to 387.5±9.33 HV0.2 and 1.58±0.05 μm, respectively. After coating with 3.7-μm thick TiN/TiO2, the average S a roughness and nanohardness of the film on the polished substrate reached 0.87±0.1 μm and 13.05±2.07 GPa while that on the EBT was equal to 1.57±0.2 μm and 9.02±2.15 GPa, respectively. However, the comparison of the coefficient of friction (COF) evolution of the substrates and the coated specimens indicated a COF decrease by about 0.18±0.03 for the coated EBT alloy as compared with the polished and EBT substrates, whose average COF were 0.45±0.04 and 0.38±0.06, respectively. A high average COF (0.68±0.16) was registered for the coated as-received alloy because of the gradual and complete wearing off of the film during the test. These results indicate that the combination of initial EBT of the Ti6Al4V alloy with PVD deposition of a TiN/TiO2 coating could substantially improve the roughness and tribological properties of the coated systems.

OPTIMIZATION OF THE GEOMETRICAL PARAMETER AND FLOW FRICTION EVOLUTION FOR TURBULENT REGIME OF THE PLAIN BEARING

OPTIMIZATION OF THE GEOMETRICAL PARAMETER AND FLOW FRICTION EVOLUTION FOR TURBULENT REGIME OF THE PLAIN BEARING